Well-known in the art is a support suitable for use in the formation of polyurethane films comprising a base, such as a sheet of paper, and a releasing layer formed on the base from a releasing material, such as silicone, alkyd resin, polypropylene and polymethylpentene. Upon production of a polyurethane film, a film-forming polyurethane in the form of a solution or melt is coated onto such a support having a releasing layer to form a film and, after solidification by drying or cooling, the film so formed is peeled from the support. When a synthetic leather, in which a polyurethane film is incorporated, is to be prepared, a polyurethane film is formed on such a support by coating a film-forming polyurethane usually in the form of a solution onto a releasing layer of the support and then solidifying it by drying with heat; an adhesive is applied onto the polyurethane film and a cloth is stuck thereto; after solidification of the adhesive by drying with heat the support is peeled away, leaving the desired synthetic leather product.
Known supports which have heretofore been used in the formation of polyurethane films suffer from various problems.
Supports having a releasing layer of silicone tend to repel a film-forming polyurethane solution when such a solution is applied onto the silicone releasing layer. Furthermore, because of the excessive releasing tendency of silicone a polyurethane film formed on the silicone releasing layer frequently peels or becomes spontaneously released from the releasing layer during the film forming process or when an adhesive is being applied onto the film.
With supports having a releasing layer of an alkyd resin, it is not easy to peel a solidified polyurethane film from the releasing layer because of the insufficient releasing ability of the alkyd resin. There are cases in which an embossed support is used for forming polyurethane films. When a support having a releasing layer of an alkyd resin is embossed, undesirable cracks and voids are frequently formed in the releasing layer. Such cracks and voids adversely affect uniform formation of polyurethane films on the one hand and greatly reduce the releasing ability of the support.
Supports having a releasing layer of polypropylene do not suffer from the problem of repelling the film-forming solution nor that of forming cracks and voids in the releasing layer when embossed. It is not easy however to peel a solidified polyurethane film from the polypropylene releasing layer because of the insufficient releasing ability of polypropylene. The polypropylene releasing layer is rather difficult to be sharply embossed. Moreover, a support comprising a base of paper and a releasing layer of polypropylene is liable to curl up in a humid atmosphere.
Japanese Patent Laid-open Specification No. 52-136901 discloses a process for producing a sheet for use in the manufacture of synthetic leathers wherein a sheet of paper is extrusion coated with polypropylene only or a copolymer of ethylene and propylene, the coating polymer having a melting point of not lower than 150.degree. C. As discussed in the preceding paragraph, however, supports having a releasing layer of polypropylene only are not satisfactory regarding their releasing ability and their ability to be embossed. As to copolymers of ethylene and propylene, practically it is difficult to prepare those having a melting point of not lower than 150.degree. C. In fact most commercially available ethylene-propylene copolymers have a melting point of substantially lower than 150.degree. C., e.g. about 140.degree. C. or lower. More importantly, a stable or constant releasing ability is not necessarily ensured by a copolymer of ethylene and propylene having a melting point above 150.degree. C.
More particularly, various supports intended for use in the formation of polyurethane films were prepared by extrusion coating sheets of paper with polypropylene only or mixtures of polypropylene and polyethylene in varied proportions melt extruded through a T-die, while varying the temperature of the coating material as extruded. The supports so prepared were tested for their properties, including the releasing ability, performance on being embossed and curl resistance. As a result, we have found the following facts.
(1) While polypropylene alone does not provide a satisfactory releasing ability, incorporation of a definite amount of polyethylene into polypropylene remarkably improves the releasing ability of the product.
(2) While both the performance on being embossed and the curl resistance of the product are unsatisfactory with polypropylene alone, incorporation of a definite amount of polyethylene into polypropylene improves these properties.
(3) For an appreciable improvement, incorporation of at least 5% by weight of polyethylene based on the total weight of the polypropylene and polyethylene is necessary. However, incorporation of polyethylene in excess of 35% by weight based on the total weight of the polypropylene and polyethylene or the use of polyethylene alone deteriorates the heat resistance of the product.
(4) A product having a remarkably improved releasing ability can be obtained by extrusion coating a paper base with a suitable mixture of polypropylene and polyethylene melt extruded at a temperature substantially lower than that conventionally practiced.